The present invention relates to a photosensitive material processing apparatus capable of processing both long photosensitive materials in the form of a roll (hereinafter referred to as "webs" when applicable) and short photosensitive materials in the form of a sheet or a leaf (hereinafter referred to as "sheets" when applicable). More particularly, the invention relates to a photosensitive material processing apparatus having an improved photosensitive material conveying mechanism.
In general, a sheet processing apparatus employs a roller conveying system. In a processing apparatus of this type, in order to develop, fix, wash and dry a sheet, the sheet is either conveyed through pairs of rollers or it is conveyed by utilizing friction between the sheet and the surfaces of rollers which are arranged staggeredly. In either case, it is necessary that the distance between adjacent conveying rollers be shorter than the minimum length of the sheet and that a number of rollers be provided to perform the above-described processes. For processing webs with such a sheet processing apparatus described above, a web to be processed is normally processed in such a manner that the web extends simultaneously through all of the processing sections, that is, from the inlet of the first processing section to the outlet of the last processing section. In this case, the web is conveyed simultaneously by a number of rollers in the processing sections. Therefore, if the speeds of the rollers are even slightly different, the different speeds result in a slackening of the web as a result of which the web meanders during conveyance. That is, the web may be folded or creased and at worst the web may be wound around the rollers with the result that the web cannot be properly conveyed.
On the contrary, a web processing apparatus is provided with means for supplying a web from a spool on which the web is wound and means for winding the processed web so that a web can be continuously processed. In this case, the web supplying means provides tension to the web by pulling the web backwardly (hereinafter referred to as "back tension" when applicable) while the web winding means provides tension to the web by pulling the web forwardly (hereinafter referred to as "pulling tension" when applicable) so that the web is conveyed under tension through all of the processing sections. An apparatus in which the web conveying speed in the final drying section is made higher than those in the earlier sections in order to increase the pulling tension is also known in the art. The apparatus is based on the same technical concept as that described above.
In an apparatus described for processing only webs and which cannot process sheets, the web is not conveyed by several different sets of conveying rollers. That is, the web is often conveyed only by the pulling tension which is applied to the web by the winding means. In this apparatus, the above-described back tension and pulling tension make it possible to convey the web satisfactorily under tension through all of the processing sections. However, in order for a conventional sheet and web processing apparatus to satisfactorily process sheets, the rollers in all of the processing sections thereof should be provided with driving or conveying power. Because of this requirement, the application of the effects of back tension and pulling tension to a photosensitive material in the processing sections has been limited.
A conventional sheet and web processing apparatus is simply a sheet processing apparatus provided with a web supplying device and a web winding device. It is desirable in such an apparatus that strong back tension and pulling tension be provided to maintain the web under tension in all of the processes. That is, the web should be processed without being slackened.
In order for, according to the above-described technique, the web to be maintained under correct tension at all of the positions in all the processing sections, the following condition must be established: EQU T1+T2&gt;n.multidot.t,
where T1 is the back tension applied to the web, T2 is the pulling tension, t is the conveying power of each roller, and n is the number of rollers.
This condition implies that the web is acted upon by a tension which is greater than the sum of the conveying powers of all the rollers. In an ordinary processing apparatus, the sum of the conveying powers is higher than the strength of a web itself. Therefore, such a condition cannot be realized. In any event, application of a high tension to a web is not desirable because it is likely to crease the web and to increase the load of the apparatus.
A sheet or a web may be conveyed by a nip conveyance technique using a number of opposed rollers. However, in this case, a problem occurs because of the relation between the width of the rollers and the width of the sheet or web. That is, when the nip conveyance technique is effected using such opposed rollers as described above, in general one roller of a pair of rollers is driven under the condition that a nipping force is applied thereto by a spring and the roller is movable in a plane perpendicular to the direction of passage of a sheet. As a sheet is conveyed through the pair of rollers, the distance between the rollers increases by the thickness of the sheet whereupon the sheet, being nipped by the spring force applied to the rollers, is conveyed onwards. If, in this operation, the sheet is conveyed with straight nip rollers, then all the nipping force would be applied to the surface of the sheet or web.
FIG. 1 illustrates the case where the width of a sheet or web 2 is comparatively large to the width of rollers 1 and 1'. FIG. 2 shows the case where the width of a sheet or web 2 is comparatively small to that of the rollers 1 and 1'. In both cases, the same total nipping force is applied. Therefore, the nip pressure per unit width increases as the width of the sheet or the web 2 decreases. When a conveyed web 2 having a small width passes near the ends of the rollers as shown in FIG. 2, then the upper roller 1' tilts as a result of which the surface pressure applied to the web 2 becomes irregular and therefore the web 2 has a tendency to meander.
As is clear from the above description, with a processing apparatus in which pairs of nip rollers are provided as straight rollers and one of each pair of rollers is depressed by spring means, a web having a small width is especially likely to meander. Moreover, the nipping surface pressure is so great that correction of such induced meandering is difficult.